Transformers influence the stability of a grid, protection coordination, harmonic operation, and the reliability of the plants in solar power plants. The most important and one of the least understood technical properties of transformer selection is the group of vectors. Errors in the selection of the vectors group may give rise to grounding, harmonic distortion, nuisance tripping and grid synchronisation problems. The good news though is that with the correct selection of vector group, inverter-utility grid integration is guaranteed.
Transformer vector groups define primary and secondary winding as well as phase displacement. It shows the type of the coupling among windings, Delta (D), Star (Y), and Zig-Zag (Z) and the difference in phase angle in clock notation.
The 360 degrees are used in the clock notation using 12 hours. The transition point is 30 degrees per hour. Dyn11 is 30 degrees behind the main and is known as the secondary or the back voltage.
Selection of a vector group has an impact on grounding, harmonic suppressions, short circuit behaviour and compatibility of inverter output.
The Dyn11 transformer vector group is chosen in most of the solar power plants that have a connection of 11kV, 22kV, or 33kV. Under this arrangement, the high-voltage winding is bypassed in Delta (D), the low-voltage winding bypassed in Star (Y) and the neutral point is brought out (n).
The ability to deal with the harmonics produced by solar inverters is the main reason why Dyn11 is so popular. Inverters work on the principle of power electronics switching that adds harmonic elements in the system. The high-voltage delta connection offers a closed circuit to third harmonic and other zero-sequence currents. This eliminates the flowing of these harmonics into the grid and enhances the quality of power.
Furthermore, the low voltage side star connection to the neutral offers effective grounding. Solar inverters systems must also have a constant neutral reference to operate correctly and coordinate protection. This balance between grounding stability and harmonic control is provided by Dyn11.
The other reason why Dyn11 is preferred in solar applications is it has a 30-degree phase shift that falls in line with the grid requirements. The majority of distribution networks are configured based on Dyn11 transformers. The compatibility of using the same group of vectors and synchronisation with the utility network is easier.
Avoiding system investigation and selecting a vector group may result in a phase offset between the operation of the transformer and those of other transformers. Mismatches may create circulating currents, over heating and inefficiency in operation.
Several transformer solar pooling stations should have similar vector group selection to prevent phase difference.
It is important to ground solar plants safely. The star-linked accompanied secondary winding with neutral may be grounded directly or by a resistor or reactor, as grid restrictions permit. This set up identifies faults correctly and ensures that earth fault protecting relays operate. Windings that are delta-connected do not have a neutral. That would complicate ground fault protection and put more danger into it.
Inappropriate neutral grounding due to the choice of vector groups to result in repeated inverter tripping as well as irregular performance of the plant in the field. Most grid connected solar projects with Dyn11 do away with these concerns.
Solar inverters generate non-linear currents that result in harmonics. Dyn11 transformers have delta windings that capture harmonics. Transformer windings, wires and switchgear may overheat without harmonic suppression. This reduces the life span of equipment and increases the cost incurred in maintenance.
In order to reduce expenses, the consultant has proposed a Yyn0 group of transformers based on the 15 MW Rajasthan solar project. Limitations were overcome by the harmonic distortion in the load testing. There was an increase in harmonic performance after using Dyn11 and the plant was stabilised.
This is an example of why Dyn11 is the standard of the solar industry.
The prevalent one is Dyn11 but in some instances, other groups of vectors are used. As an example in large pooling substations with step-up transformers coupling 33kV to 132kV or 220kV grids, a group of Ys with N being a prime number (e.g. YNd1 or YNd11) can be designed depending on grid utility requirements. System studies can suggest alternative settings in floating solar or special industrial captive plants. They are, however, project specific decisions that are arrived at following load flow and fault analysis.
Dyn11 is still the recommended and proven layout in the case of standard 11kV or 33kV solar evacuation transformers.
Where several transformers are used in parallel in solar parks, matching vector groups are required. When two transformers of different vector group attempt to run simultaneously, serious circulating currents may arise as a result of phase angle difference.
Dyn11 guarantees standardisation and ease of parallel operation. This is especially essential in huge solar plants with multiple inverter blocks that connect to a shared pooling station.
Projects that have suffered unwarranted delays merely because a mismatch of vectors group was only realised late in the commissioning. Such setbacks are prevented by early coordination of engineering.
Dyn11 offers a delta connection on the high voltage side, which suppresses harmonics as well as preventing the entry of zero-sequence currents into the grid. The low-voltage side of the star is connected to the grounded neutral and allows the inverter to function properly. The 30 degree phase shift ensures that most distribution networks are compatible.
This combination has rendered Dyn11 technically reliable, grid-friendly, and economically feasible in use in solar power applications.
An appropriately chosen group of vectors will increase the life of a transformer and minimise disruptions in the operation. The lifetime of solar plants is calculated at 25 years, and the layout of the transformers is directly related to the realisation of the goal.
The transformer works effectively under optimal solar generation conditions, particularly during that of high ambient temperature, when using Dyn11 with appropriate impedance design, harmonic considerations and temperature rating.
Choosing an incorrect group of vectors may allow the initial cost to be saved at the cost of harmonics problems and protection failures as well as recurring outages.
Dyn11 has been the most popular in solar transformers due to its ability to suppress harmonics, ground well and has compatibility with grid systems. Its 30-degree phase-shift delta-star form is appropriate to the medium-voltage solar power evacuation.
Other types of the vector groups can be used under special circumstances, however, Dyn11 is used as a standard of 11kV and 33kV solar transformers. Knowledge and selection of a vector group provides stability, quality of power and reliability of solar plants.
Even such a simple design aspect as the vector group can influence the stability of the PV systems. To ensure that the transformer can sustain the plant over decades, it is important to start with proper arrangement that ensures the transformer supports the plant.
Related Link: WHY ARE INVERTER DUTY TRANSFORMERS PREFERRED IN SOLAR PLANTS?
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